JPL SBDB query sometimes fails when ``alternate_id=True``

While testing for random asteroids, I found KeyError happens for few cases when ``alternate_id=True`` (if I turn this off, it works fine for my test cases).

Sample code:
```python
from astroquery.jplsbdb import SBDB
obj = SBDB.query('331471', phys=True, alternate_id=True)
```
Error:
```
---------------------------------------------------------------------------
KeyError Traceback (most recent call last)
in
----> 1 obj = SBDB.query('331471', phys=True, alternate_id=True)
2 obj

~/anaconda3/lib/python3.7/site-packages/astroquery-0.3.10.dev5703-py3.7.egg/astroquery/utils/class_or_instance.py in f(*args, **kwds)
23 def f(*args, **kwds):
24 if obj is not None:
---> 25 return self.fn(obj, *args, **kwds)
26 else:
27 return self.fn(cls, *args, **kwds)

~/anaconda3/lib/python3.7/site-packages/astroquery-0.3.10.dev5703-py3.7.egg/astroquery/utils/process_asyncs.py in newmethod(self, *args, **kwargs)
27 if kwargs.get('get_query_payload') or kwargs.get('field_help'):
28 return response
---> 29 result = self._parse_result(response, verbose=verbose)
30 self.table = result
31 return result

~/anaconda3/lib/python3.7/site-packages/astroquery-0.3.10.dev5703-py3.7.egg/astroquery/jplsbdb/core.py in _parse_result(self, response, verbose)
205 raise ValueError(src['message'] + ' ({:s})'.format(src['code']))
206
--> 207 src = self._process_data(src)
208
209 # add query uri, if desired

~/anaconda3/lib/python3.7/site-packages/astroquery-0.3.10.dev5703-py3.7.egg/astroquery/jplsbdb/core.py in _process_data(self, src)
314 # re-run this function on leaf dicts
315 elif isinstance(val, dict):
--> 316 res[key] = self._process_data(val)
317
318 # use leaf scalars (and apply units, where applicable)

~/anaconda3/lib/python3.7/site-packages/astroquery-0.3.10.dev5703-py3.7.egg/astroquery/jplsbdb/core.py in _process_data(self, src)
267 # try to convert list of strings to array
268 for i in range(len(val)):
--> 269 if isinstance(val[i][field], bytes):
270 val[i][field] = val[i][field].decode(
271 'utf-8')

KeyError: 'des'
```

As seen in the error lines, I used python 3.7 and astroquery 0.3.10.dev5703.

Reproduce the sample SBDB.query('331471', phys=True, alternate_id=True) call, then start in astroquery/jplsbdb/core.py at _parse_result and _process_data, where the traceback shows the KeyError. Trace how alternate identifiers are processed for this response. Done means the sample query succeeds with alternate_id=True without raising KeyError, while the existing behavior remains intact when it is false.

Written by the indexing model from the issue text.